Announcing your plans to others satisfies your self-identity just enough that you’re less motivated to do the hard work needed.

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It may seem unnatural to keep your intentions and plans private, but try it. If you do tell a friend, make sure not to say it as a satisfaction (“I’ve joined a gym and bought running shoes. I’m going to do it!”), but as dissatisfaction (“I want to lose 20 pounds, so kick my ass if I don’t, OK?”)